To flip the current card, click it or press the Spacebar key.
To move the current card to one of the three colored boxes, click on the box. You may also
press the UP ARROW key to move the card to the "Know" box, the DOWN ARROW key to
move the card to the "Don't know" box, or the RIGHT ARROW key to move the card to the Remaining box.
You may also click on the card displayed in any of the three boxes to bring that card back to the
center.